This research is concerned with the study of the structure and function of the enzyme creative kinase. Creatine kinase plays an important role as a regenerator for adenosine triphosphate (ATP) in muscle energy metabolism. The enzyme specifically catalyzes the reaction (ATP plus Creatine yields reversibly Creatine phosphate plus ADP) in the presence of divalent cations such as Mg2 plus and Mn2 plus. The study consists of two parts, namely, (1) Determination of the three-dimensional structure of the enzyme from muscle by x-ray crystallographic technique, and (2) Analysis of the chemical, spectral and kinetical properties of the enzyme modified with chromophoric organomercurials which are specific for thiol groups. Once the crystal structure of the enzyme has been solved, the Fourier difference method will be used to study the binding of substrates. X-ray analysis of substrates binding will provide further detailed structural and stereochemical bases for the conformational change which seems to occur upon binding, as indicated by published data obtained in solution. The role of the chromophoric organomercurials, besides being possible heavy atom derivatives, in the study of the enzyme in solution, is based on the following potential uses: chromophoric probes for thiol groups in the enzyme, kinetic studies of reactivity of thiol groups, and "reporter groups" - indicators of perturbations in the enzyme on which they are attached when substrates are bound. A long-range objective includes a structural and functional comparison of other creative kinase isoenzymes to be based similarly on the two-part study. Published data have indicated differences among isoenzymes. Future corresponding studies on the enzyme isolated from dystrophic muscle will likely be of interest to those concerned with the problem of propressive muscular dystrophy.